Having played this the other night, I have some small(read, tending for huge) suggestions to make.
The whole map area seems too cramped. The only space available for battle is right besides the point, and all the rest of the area can be dominated with a pyro or heavy. My suggestion:
The green rectangle is an extension of the battle area for the main cap point. it should be at the same level or a bit above, have some random walls and props to avoid being empty and be directly connected. This would allow for engineers to setup sentries besides the balcony and for classes such as spies and scouts to navigate more freely.
The red rectangles are basically the underground passage you already have, just noting that they should probably stay, although being connected to the green area would help.
The blue rectangles is just an idea, allowing both teams to emerge overlooking the point, to avoid it being locked down by a sentry, just add a small tunnel that leads into the flipped control center.
Some stupid random ideas:
The cap finish should involve the Death Star firing against this.
When you open the airlocks it should pull all players in the balcony areas. Thus being a viable strategy to get an engineer away from his sentry.